1. Field of the Invention
The present invention relates to an optical position assessment apparatus and method which may be used in lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements which serve to generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one pass, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It will be appreciated that, whether or not a lithographic apparatus operates in stepping or scanning mode, it is vital that the patterned beam is directed onto the appropriate target portion of the substrate surface. In many circumstances multi-layer structures are built up on the surface of the substrate as a result of a sequence of lithographic processing steps. It is of course vital that the successive layers formed in the substrate are correctly in register with each other. Thus, great care is taken to ensure that the position of the substrate relative to the beam projection system is accurately known.
Various techniques are used to determine the position of a substrate relative to the beam projection system. Generally, the substrate has formed upon it alignment marks that are arranged around the periphery of areas of the substrate onto which active circuit components or the like are to be formed. These marks are located to provide reference points relative to which the position of target portions on the substrate are determined. Ideally, the alignment marks are detected optically using the beam projection system, which is also used to project patterns onto the substrate. Such a “through the lens” or TTL approach to the problem of locating alignment marks has the advantage that the position measurement location is the same as the image formation location. Thus errors are minimized.
Position determining accuracy, which term is used to include x, y offset, rotation, and magnifications etc., is a function of the spacing between alignment marks. The smaller that spacing the better, as it reduces the extent to which accuracy depends upon the accuracy of control of movements of the substrate relative to the beam projection system. There are circumstances, however, where it is difficult to avoid large spacings between alignment marks. For example, large liquid crystal display (LCD) panels are now envisaged with diagonal sizes (generally quoted in inches, e.g., 32, 42, 60 inches etc.) resulting in edge dimensions of the order of one meter. All of the surface of such a device is occupied by active components and therefore there is no space available for alignment marks except around the periphery of the panels. This means that the spacing between the alignment marks is of the same order as the edge dimension of the panel. This makes it very difficult to maintain positional accuracy across the whole of the area of the panel.
Conventionally, a lithographic apparatus delivers a projection beam to the substrate through a lens assembly in which each of the lenses is arranged in series along the beam projection direction. The lens component closest to the substrate is a single lens through which all of the projection beam passes. It has been proposed, however, to use an alternative design approach in which again a series of lenses are arranged along the projection beam path, but the lens component closest to the substrate is in the form of a two dimensional array of small lenses. Each of those small lenses focuses a respective part of the projection beam onto a respective part of the substrate. Systems in accordance with this alternative design are generally referred to as microlens array imaging systems or MLA systems. In MLA systems, it has not been possible to use conventional “through the lens” approaches to the detection of alignment marks on a substrate to be exposed. As a result, it has been considered necessary to provide separate metrology systems for alignment purposes.
Therefore, what is needed is an improved position assessment (measuring) apparatus and method which can be used in a microlens array imaging system.